Supported CuO + Ag/partially stabilized zirconia catalysts for the selective catalytic reduction of NOx under lean burn conditions

Article abstract of DOI:10.1006/jcat.2001.3194

The catalytic performances of copper oxidic species supported on mesoporous zirconia supports stabilized by alkaline earth cations were studied for the NO oxidation reactions to NO₂ and NO_x SCR in excess oxygen. In the reaction of NO

Full text of DOI:10.1006/jcat.2001.3194

Journal of Catalysis 200, 131–139 (2001)
doi:10.1006/jcat.2001.3194, available online at http://www.idealibrary.com on
Supported CuO + Ag/Partially Stabilized Zirconia Catalysts for the
Selective Catalytic Reduction of NOx under Lean Burn Conditions
2. Catalytic Properties
,
,1
Vladislav A. Sadykov, † R. V. Bunina, G. M. Alikina, A. S. Ivanova, T. G. Kuznetsova,
S. A. Beloshapkin,† V. A. Matyshak,‡ G. A. Konin,‡ A. Ya. Rozovskii,§ V. F. Tretyakov,§
T. N. Burdeynaya,§ M. N. Davydova,§ J. R. H. Ross,¶ and J. P. Breen¶
Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, Lavrentieva 5, Novosibirsk 630090, Russia; †Novosibirsk State
University, Pirogova 1, Novosibirsk 630090, Russia; ‡Semenov Institute of Chemical Physics RAS, Moscow, Russia; §Topchiev Institute
of Petrochemical Synthesis RAS, Moscow, Russia; and ¶University of Limerick, Limerick, Ireland
Received October 6, 2000; revised January 30, 2001; accepted February 2, 2001; published online April 11, 2001
INTRODUCTION
The catalytic performances of copper oxidic species supported
Zirconia-supported transition metalcationsare known to
possess a reasonably good catalytic activity in the reactions
of NOx-selective reduction by hydrocarbons in the excess
of oxygen (1–6). In earlier studies, transition metal cations
were either incorporated into the bulk of zirconia via the
coprecipitation route or supported on monoclinic/tetrago-
nal modifications of ZrO2 with a moderate (up to 60 m2/g)
surface area. In the reaction of NOx-selective reduction
by propylene, catalytic performance of those systems was
found to be comparable to that of cation-exchanged ZSM-5
(1–3, 6). However, in the case of propane as reducing agent,
those systems were only moderately active (1–3). For NOx
reduction by long-chain hydrocarbons such as decane, a
good performance was obtained only in the case of cop-
per supported on sulfated zirconia (4, 5). Hence, for differ-
ent types of hydrocarbons as reducing agents, performance
of zirconia-supported transition metal cations seems to be
quite different. In part, it can be assigned to specificity
of reaction mechanism depending upon the mode of hy-
drocarbon activation and subsequent transformation of in-
termediates on different types of isolated/clustered cations
known to be present on the surface of those systems (1–7).
Since coordination and clustering degree of supported tran-
sition metal cations are well known to depend upon the
support chemical nature, bulk/surface structure and its dis-
ordering, specific surface area and pore structure, system-
atic studies of the effect of those parameters on transition
metal cations catalytic properties in the NOx selective re-
duction by different hydrocarbons is certainly of a great
interest from the fundamental and applied point of view. In
our previous communication (7), mesoporous zirconia sam-
plesstabilized byalkaline-earth cationswith supported cop-
per cations and silver were synthesized and characterized
on mesoporous zirconia supports stabilized by alkaline earth
cations were studied for the reactions of NO oxidation to NO2
and the selective catalytic reduction (SCR) of NOx in excess oxy-
gen. Three different types of hydrocarbon reductants were stud-
ied, namely, propane, propylene and the long-chain hydrocarbon
decane. For comparison, the performance of CuO and CuO + Ag
supported on alumina for the SCR of NOx by propane was also
studied. This data was analyzed and discussed with reference to
the information concerning the surface properties of these systems
presented in Part 1 of this series. In general, for the same system,
the maximum level of NOx conversion and temperature of maxi-
mum NOx conversion is strongly dependent upon the type of re-
ducing agent. No universal relation was found between the sur-
face density of coordinatively unsaturated cations able to activate
hydrocarbons and the activity of the catalysts for the SCR reac-
tion. In part, it can be explained by blocking the surface sites in
reaction media either by strongly bound nitrate species or by coke.
Addition of silver was found to be of significance only in the case
of strong interaction between the metallic and oxidic components.
Such a factor as the strength of oxygen and nitrate complexes bond-
ing with the surface copper/copper–silver oxidic species depending
upon the type of support and methods of samples synthesis appears
to be the most important for performance of the systems studied in
NOx-selective reduction by hydrocarbons in the presence of excess
c
oxygen.
2001 Academic Press
Key Words: NOx selective reductions by propane; propylene; de-
cane; supported copper oxide; promotion by silver; mesoporous sta-
bilized zirconia supports.
1 To whom correspondence should be addressed at Boreskov Insti-
tute of Catalysis, Siberian Branch of the Russian Academy of Sciences,
Lavrentieva 5, Novosibirsk 630090, Russia. Fax: (7-3832)-343-056. E-mail:
sadykov@catalysis.nsk.su.
131
0021-9517/01 $35.00
c
Copyright
2001 by Academic Press
All rights of reproduction in any form reserved.

132
SADYKOV ET. AL.
by using a combination of such sophisticated methods as
XPD, TEM, EXAFS, SAXS, FTIRS of adsorbed CO, and
TPD of NOx. The aim of the present paper is to character-
ize the performance of these systems in the SCR of NOx
by hydrocarbons using three different types of reductant
and to elucidate the features responsible for the catalytic
properties of these systems.
METHODS
The methods of sample synthesis and their properties
are described in the first paper of the series (7). The prop-
erties of the catalysts for the NOx SCR and NO oxidation
to NO2 by O2 reactions were tested in flow microreactors
using procedures described previously (8–11) at GHSV
4000/h–12,000/h (0.1% C3H8 + 0.1% NO + 1% O2 in
He, feed 1), 18,000/h (0.2% C3H8 + 0.2% NO + 2.5% O2
in N2, feed 2), 18,000/h (0.2% C3H6 + 0.2% NO + 2.5% O2
in N2, feed 3), 11,200/h (0.05% C10H22 + 0.15% NO in air,
feed 4), and 30,000/h (1% NO + 1% O2 in He, feed 5). The
error in determining NO conversion for feed 5 using FTIRS
[10] was found to be ca. 2 rel% , whereas for feeds 1–4,
where GC and a NOx chemiluminescence analyzer were
used (9, 11), corresponding error was found to be around
5 rel% .
FIG. 2. Temperature dependence of NO oxidation into NO2 on
Ag + CuO/MeO–ZrO2 samples; feed 5.
of these systems determined primarily by the size of substi-
tuting cation (7). In addition, at the low content of alkaline-
earth cations used here, in the surface layer, the energy of
the Zr–O bond estimated by the ratio of the ion currents
ZrO + /Zr + was found to increase from Ba- to Ca-modified
samples (8). Hence, at high temperatures, the rate of NO
oxidation into NO2 appears to be determined by rather
strongly bound oxygen, which is also responsible for the
relative stability of such possible intermediates in NOx HC-
SCR as nitrite–nitrate species (9, 10). At low temperatures,
molecular oxygen species could be important for NO oxi-
dation. At least, for BaO–ZrO2 sample, the decrease of NO
conversion with temperature can be explained by desorp-
tion of these weakly bound forms.
RESULTS
1. No Oxidation into NO2
Figure 1 shows typical results obtained for pure supports
and supported Ag–CuO systems. For pure supports, the
rates of NO oxidation are rather low, being only slightly
dependent upon the type of substituting cation. Even at
the highest temperatures used in our experiments, NO
conversion into NO2 is lower than the equilibrium value
(10). At high temperatures, the rate of NO oxidation de-
creases in the order BaO–ZrO2 > SrO–ZrO2 > CaO–ZrO2,
which correlates with the degree of structural disordering
For Ag + CuO/MeO–ZrO2 samples, the rates of NO ox-
idation are strongly increased as compared with supports
(Fig. 2). At low temperatures, conversion decreases in the
order Ag + CuO/CaO–ZrO2 > Ag + CuO/SrO–ZrO2
>
Ag + CuO/BaO–ZrO2. This order does not correlate with
adsorption properties of these systems with respect to CO
or NO (7). At high temperatures, NO conversion is de-
termined by equilibrium being insensitive to the nature of
samples. Hence, at temperatures exceeding 350 C, both the
NO oxidation into NO2 and the NO2 decomposition reac-
tions are very fast. This agrees with the TDP data, indicating
that for these samples, Tmax of nitrates decomposition are
usually in the range of 350–380 C (7).
2. Selective NOx Reduction by Propane
For pure supports, the results are presented in Fig. 3.
In the first approximation, the rates of NO reduction and
propane oxidation correlate with the surface disordering
of these samples (7) and the rate of NO oxidation into
NO2 (vide supra). Hence, for modified zirconias, the rate of
NO selective reduction can be determined by the bonding
strength of nitrate species affecting their reactivity, as was
suggested in (8, 9), and/or the rate of propane activation,
FIG. 1. Temperature dependence of NO oxidation into NO2 on meso-
porous zirconia supports stabilized by alkaline-earth cations; feed 5.

HC-SCR OF NOx ON CuO (+Ag)/MeO–ZrO2
133
FIG. 5. Temperature dependence of NO selective reduction into N2
by propane in the excess of oxygen (feed 2, GHSV 18,000/h) on MeO–
ZrO2 samples loaded with 0.2 wt% copper (ion exchange route, 1–3) or
with 2wt% Ag + 5wt% CuO (consecutive impregnation route, 4–6). Me =
Ca (1, 4), Sr (2, 5), and Ba (3, 6).
FIG. 3. Temperature dependence of NO conversion into N2 and
propane oxidation into COx on mesoporous MeO–ZrO2 samples. Me =
Ca (1), Ba (2), and Sr (3). Feed 1, GHSV 4000/h.
(200–250 C) temperatures (Fig. 5). It seems to be mainly
explained by the decrease of the oxygen bonding strength
due to clustering and formation of mixed oxidic species (7),
leading to more efficient activation/conversion of propane
(Fig. 6). In addition, the bonding strength of bridging and/or
bidentate nitrates is also decreased for these systems (7),
thus helping to increase the low-temperature activity. In-
deed, the Ag + CuO/BaO–ZrO2 sample, which exhibits
the highest level of low-temperature activity, is character-
ized by the lowest thermal stability of nitrate surface com-
plexes (7). Moreover, for this sample, the lowest frequency
of the carbonyl band was observed, implying pronounced
interaction between the copper and silver oxidic species (7).
For Ag + CuO/MeO–ZrO2 samples prepared via pho-
todeposition, as compared with samples prepared via suc-
cessive impregnation, the range of the efficient operation
is shifted to higher (350–450 C) temperatures (Figs. 7 and
8). For the former samples, the content of copper and silver
(in the range of 0.2% Ag + 3% CuO) is somewhat lower
both being determined by the bonding strength of oxygen
in the surface layer.
For CuO/MeO–ZrO2 systems prepared via the impreg-
nation route, in the first approximation, the rates of NO-
selective reduction by propane are comparable (Fig. 4), this
is in agreement with the conclusions of Pietrogiacomi et al.
(6) about the low sensitivity of the turn over rates of SCR
reactions for copper-supported zirconia samples. Subtle dif-
ferences, namely, a high rate of propane combustion for the
CuO/CaO–ZrO2 system leading to decreased NOx reduc-
tion, especially at enhanced (>350 C) temperatures, can be
explained by the highest surface density of coordinatively
unsaturated Cu + centers (7), leading to a high rate of the
radicals generation.
In the case of Ag + CuO /MeO–ZrO2 systems prepared
via the consecutive impregnation, a pronounced interac-
tion between the metallic and oxidic components forming
either mixed clusters or even microphases of interaction
(7) helps to shift the maximum of NO conversion to lower
FIG. 6. Temperature dependence of propane conversion into CO2 on
MeO–ZrO2 samples loaded with 0.2 wt% copper (1–3) or with 2 wt%
Ag + 5 wt% CuO (4–6). Feed 2, GHSV 18,000/h. Me = Ca (1, 4), Sr (2,
5), and Ba (3, 6).
FIG. 4. Temperature dependence of NO conversion into N2 and
propane oxidation into COx on MeO–ZrO2 samples loaded with 5 wt%
of CuO (impregnation route). Me = Sr (1), Ba (2), and Ca (3). Feed 1,
GHSV 4000/h.

134
SADYKOV ET. AL.
partial oxidation, which can be assigned to a higher bonding
strength of oxygen in this case. According to our previous
experience (7, 12), a higher bonding strength of oxygen
with supported copper oxidic species is usually reflected
in their lower reducibility by CO at low (77 K) tempera-
tures and higher frequency of bands corresponding to com-
plexes of CO with Cu + cations of those species. Indeed, for
Ag + CuO/MeO–ZrO2 samplesprepared via photodeposi-
tion, at fullCO coverage, those bandsare situated at 2115–
2117 cm 1, which is certainly higher than 2100 cm 1 posi-
tion for samples prepared via the consecutive impregnation
(7). Rather close catalytic performance of catalysts pre-
pared via the photodeposition route (Fig. 7) agrees well
with comparable surface densities of coordinately unsatu-
rated Cu + centers probed by this test molecule (7). Further,
a somewhat higher performance of Sr-stabilized zirconia
supported catalyst correlates with a higher intensity of cor-
respondingcarbonylband (7). At present, for these systems,
the effect of silver (if any) is not apparent, probably, due to
independent location of copper and silver oxidic species on
the surface (vide infra).
FIG. 7. Temperature dependence of NO conversion into N2 (solid
lines) and propane oxidation into COx (dashed lines) on MeO–ZrO2 sam-
ples loaded with silver and copper via consecutive photoassisted deposi-
tion route. Sample composition: 0.8% Ag + 3 wt% CuO/BaO–ZrO2 (1),
0.2% Ag + 3.1 wt% CuO /CaO–ZrO2 (2) and 0.2 wt% Ag + 2.5 wt%
CuO/SrO–ZrO2 (3). Feed 1, GHSV 14,000/h.
as compared with that for the latter (2% Ag + 5% CuO).
Hence, a lower degree of copper oxidic species clustering
and a less pronounced effect on their properties of a smaller
amount of loaded silver is expected for samples prepared
via the photodeposition route. Though more detailed and
systematic studies are certainly required to elucidate all
fine structural features of systems prepared by using pho-
todeposition, in the first approximation, a higher intensity
of Cu + –CO and Cu2 + –CO bands revealed for these sys-
tems and a higher frequency of Cu + –CO band (7) certainly
agrees with conclusion about higher dispersion and lower
clustering degree of copper oxidic species for these sam-
ples. A lower rate of propane oxidation at relatively low
(<350 C) temperatures for sample prepared via the pho-
todeposition route (Fig. 8) implies a lower ability of less
aggregated copper oxidic species to activate propane by its
In the case of alumina as a support (Fig. 9), for the same
method of photoassisted deposition of active components,
FIG. 9. (a) Temperature dependence of NO conversion into N2 on
alumina loaded with copper (1, 3) or silver + copper (2, 4) via consec-
utive photoassisted deposition route. Copper was supported from nitrate
solution (1, 4) or from acetate solution (3, 4). Copper oxide content is
equal to 0.7 wt% (1, 2) or to 0.5 wt% (3, 4). Silver content is equal to
0.6 wt% (2) or to 0.3 wt% (4). Feed 1, 12,000/h. (b) Temperature depen-
dence of propane oxidation into CO2 on alumina loaded with copper or
silver + copper via the consecutive photoassisted deposition route. Curve
designation is the same as in (a). Feed 1, 12,000/h.
FIG. 8. Comparison of performance of Ag, CuO-containing samples
prepared on BaO–ZrO2 support via photoassisted deposition (1, 2) and
impregnation (3, 4) routes. 1, 3-NOx into N2; 2, 4-propane into CO2. Active
components: 0.8% Ag + 3 wt% CuO (1, 2) and 2 wt% Ag + 5 wt% CuO
(3, 4). 1, 2, Feed 1; 12,000/h; 3, 4, feed 2, 18,000/h.

HC-SCR OF NOx ON CuO (+Ag)/MeO–ZrO2
135
operating temperatures are even higher than for zirconia- posited on the free sites of support not affecting the prop-
supported samples. Thus, at temperatures in the range of erties of copper oxidic species (7). Since silver cations are
350–400 C, where performance of zirconia-supported sam- known to strongly retain nitrate species, they are not able to
ples reaches a maximum, the activity of alumina-supported activate propane up to 450 C (10, 14, 15), and, hence, par-
samples is still low. Note that at middle ( 400 C) tempera- ticipate in the SCR of NOx. As a result, NOx and propane
tures, for samples without silver, the increase of the copper conversion were not changed appreciably after silver addi-
content from 0.5 to 0.7% (samples prepared using copper tion (Fig. 9).
nitrate and acetate solutions, respectively) has a very little
For a sample with a higher content ofcopper (0.7% ), a de-
effect on the degree of NO conversion, while considerably gree of copper clustering is somewhat higher, which follows
enhancing propane oxidation. At temperatures exceeding from a lower intensity of Cu + –CO carbonyl band (7). In
480 C, the degree of NO conversion is lower for samples thiscase, silver ismainlydeposited on copper clustersblock-
with a higher copper content, probably, due to a higher rate ing coordinatively unsaturated copper cations (7). As the
of propane combustion. The results for higher (1 wt% ) cop- result, the rate of propane consumption was strongly hin-
per loading on alumina presented in (9) as well as results dered (Fig. 9), which decreased NO conversion at temper-
obtained within this work for 1.5% CuO/alumina sample, atures <450 C as well. However, at higher temperatures,
not shown here for brevity, demonstrate that, in every case, the effect of silver addition on NO conversion was certainly
increasing the copper content does not help to increase beneficial in this case, probably due to conjugation between
the low-temperature performance and even deteriorates functions played by metallic and oxidic components lead-
the high-temperature performance. Hence, for copper/alu- ing to a more efficient generation of such intermediates as
mina catalysts, a shift of the range of efficient performance organic nitrocompounds [10].
in the HC–NOx reactions to higher temperatures as com-
pared with copper/zirconia systems could not be explained
by a lower loading alone. Moreover, as was shown in (7), for
samples with copper loading in the range of 0.5–2 wt% , the
density of Cu + centers probed by the FTIRS of adsorbed
CO molecules and related to the unit weight of copper is
comparable for both supports. Hence, the reason for the
different activity range of alumina- and zirconia-supported
catalysts cannot be explained exclusively by a different de-
gree of copper clustering dependent upon its surface con-
centration. From the mechanistic point of view, the most
essential difference seems to be the bonding strength of
nitrite–nitrate species, which were shown to be intermedi-
ates in the reaction of NOx reduction by propane (9, 13, 16,
17). Indeed, for the copper/alumina system, the TPD max-
imum corresponding to nitrates decomposition is situated
at 450 C (9), while for CuO /zirconia it is shifted to lower
(360–380 C) temperatures (7). Hence, the temperature of
nitrate complexes decomposition practically coincides with
the ranges of the most efficient performance of these sys-
tems, which appears to be the general feature of reactions
with participation of strongly bound intermediates (9). As
was shown in (7, 12), among alumina-supported copper ox-
idic species, two-dimensional clusters dominate, which are
nearly absent for the case of zirconia support. In these clus-
ters, all oxygen atoms are strongly bound with two copper
cations (12). As the result, bridging or bidentate nitrates
located at these clusters have a higher bonding strength (9)
and lower reactivity as compared with ad-NOx species on
CuO /zirconia catalysts, where isolated copper species and
three-dimensional copper oxidic clusters appear to domi-
nate (7).
3. Selective NOx Reduction by Propylene
The data presented in Figs. 10 and 11 imply that
for the systems considered here, in the first approximation,
the catalytic properties in the reactions of NOx reduction
by propane and propylene are similar. Thus, the high-
est low-temperature activity in both reactions is observed
for Ag + CuO/BaO–ZrO2 system (vide supra). However,
some difference is observed for samples with a low copper
content without silver: for the case of propylene as reduc-
tant, due to its easier activation as compared with propane,
silver is not required to achieve a high level of activity at
low/middle temperatures. As was shown in our previous
work by using in situ IR studies (11), two routes of NOx
FIG. 10. Temperature dependence of NO selective reduction into N2
by propylene in the excess of oxygen (feed 3) on MeO–ZrO2 samples
loaded with 0.2 wt% CuO (ion exchange route, 1–3) or with 2 wt% Ag +
5 wt% CuO (consecutive impregnation route, 4–6). Me = Ca (1, 4), Sr (2,
The effect of silver addition was quite different for the
two copper/alumina samples studied here. When loadings
of both copper and silver are low, silver appears to be de- 5), or Ba (3, 6). Feed 3, 18,000/h.

136
SADYKOV ET. AL.
FIG. 13. Temperature dependence of NO selective reduction into N2
by decane in the excess of oxygen on MeO–ZrO2 samples loaded with
2% Ag + 5 wt% CuO (impregnation route) as compared with 2 wt%
Ag/alumina sample. Feed 4, 11,200/h.
FIG. 11. Temperature dependence of propylene conversion into CO2
on MeO–ZrO2 samples loaded with 0.2 wt% CuO (1–3) or with 2 wt%
Ag + 5 wt% CuO (4–6). Feed 3, 18,000/h. Me = Ca (1, 4), Sr (2, 5), or Ba
(3, 6). Feed 3, 18,000/h.
saturated Cu + centers due to aggregation of copper cations
into bulky copper oxidic species (7). In this case, the rate of
decane activation on those sites is expected to be too low
to ensure a reasonable activity. Surface coking/carboniza-
tion ofthe surface ofzirconia-supported samplesdepending
upon the density of relatively strong Lewis/Brønsted acid
sites is worth considering as a possible reason for deactiva-
tion as well.
reduction by propylene are realized in the case of supported
copper systems: a first one includes interaction of propylene
with adsorbed nitrite-nitrate species (similar to that occur-
ring in the case of propane as reductant (13, 16, 17), while
the second one proceeds by NO interaction with a coke de-
posited on the support. Hence, the second route, which is
expected to be the most essential in the case of low copper
loadings, is less sensitive to the state of supported copper
oxidic species.
Silver addition to copper via impregnation greatly im-
proves performance of catalysts on all supports (cf. Figs. 12
and 13). Comparison with performance of a specially pre-
pared Ag/alumina catalyst (Fig. 13) demonstrates that
this enhancement of activity is of a nonadditive nature.
The increase of performance due to modification by sil-
ver correlates with the increase in the density of coor-
dinatively unsaturated Cu + sites (7), which agrees with
suggestion about the role of these centers in the de-
cane activation (vide supra). Indeed, the order of activity
observed here (Ag + CuO/BaO–ZrO2 > Ag + CuO/SrO–
ZrO > Ag + CuO/CaO–Zr) is the same as for the case of
propane as reducing agent (vide supra), which appears to
be activated primarily on coordinatively unsaturated Cu +
sites (17). Another important factor worth considering is
the decrease of the bonding strength and increase of the
bidentate and bridging nitrates reactivity due to silver addi-
tion (7), which certainly help to ensure a high performance
of modified systems in the reaction of NOx reduction by
decane. At last, decrease of the oxygen bonding strength in
silver-modified samples (7) could help to avoid the surface
coking suggested above as one of the reason for the absence
of activity for some nonpromoted samples.
4. Selective NOx Reduction by Decane
For this reaction, the catalytic behavior of systems con-
sidered here is more complicated. There is certainly a trend
in increasing the activity with the increase in the copper
content (Fig. 12); nevertheless, the reason for the absence
of activity for systems having a good performance in the
NOx reduction by propane is not so clear at present. The
absence of activity for a 5% CuO/BaO–ZrO2 sample ap-
pears to correlate with a low density of coordinatively un-
DISCUSSION
1. The Role of Support
FIG. 12. Temperature dependence of NO-selective reduction into N2
by decane in the excess of oxygen on MeO–ZrO2 samples loaded with
0.2 wt% CuO (ion exchange route, 1–3) or with 5 wt% CuO (impregnation
route, 4–6). Me = Sr (1, 5), Ca (2, 6), or Ba (3, 4). Feed 4, 11,200/h.
The role of support appears to be mainly determined
by its ability to affect the type and size of various oxidic
species due to the known effect of interaction between the

HC-SCR OF NOx ON CuO (+Ag)/MeO–ZrO2
137
oxide species and supports (11). At the same level of cop- ide (vide supra), when such species dominate, appreciable
per loading and the same preparation procedure, partially propane conversions are achieved at temperatures in the
stabilized zirconias as supports favor domination of such range of 300–350 C. At low (up to 2 wt% ) loading of silver
species as isolated copper cations and small reactive three- on alumina, silver exists as oxidic clusters stabilized by sup-
dimensional clusters, while for alumina, two-dimensional port (19). These species have a moderate ability to activate
copper oxidic clusters are the most abundant (7, 11). As a hydrocarbons. Thus, for 2 wt% Ag/alumina catalyst, at
result, for copper oxidic species, on average, the bonding GHSV 13,000/h–18,000/h, even in mixtures with a great
strength of oxygen and reactive nitrite–nitrate intermedi- excess of oxygen ( 10% ), detectable ( 10% ) propane con-
ates is higher for the case of alumina-supported systems, versions were achieved only at temperatures 400–450 C
which is reflected in higher operating temperatures in the (14, 15). Hence, for catalysts with silver and copper oxidic
NOx selective reduction by hydrocarbons in the excess of species independently distributed on the support, nonaddi-
oxygen.
tive effects in the hydrocarbons activation are not expected
For zirconia partially stabilized by the alkaline-earth as well.
cations studied here, the nature of the guest cation only
(iii) The strength of the reactive nitrite–nitrate species
moderately affects the structural features of supported cop- bonding with silver cations is comparable or even higher
per oxidic species, especially in the case of higher copper (14) than with copper or alumina cations (9, 10). Hence, at
loadings. As a result, in the first approximation, in the reac- temperatures of supported copper catalysts efficient per-
tions of selective NO reduction by propane and propylene, formance, the independent route of NOx reduction via
catalytic properties of copper on these supports are compa- propane interaction with ad-NOx species strongly bound
rable. However, stronger effects are observed in the case of with silver cations is not expected to appreciably increase
decane as reducing agent, which certainly deserve further the degree of NOx conversion.
studies.
When silver clusters are juxtaposed on copper oxidic
At the same operational parameters (space velocities and
species, as is revealed in this work for the case of alumina-
feed composition), the maximum degree of NOx reduction
supported catalysts, catalytic properties are changed in
is comparable for alumina- and zirconia-supported copper
a complex fashion: the rate of propane oxidation is de-
catalysts. However, from the practical application point of
creased in all the temperature range studied, the low-
view, lower operating temperatures for zirconia-supported
temperature NOx conversion is decreased as well, while the
samples appear to be more attractive.
high-temperature conversion is improved. These features
are explained by the preferential location of silver species
2. The Role of Interaction between the Metallic
and Oxidic Components
on coordinatively unsaturated copper cations thus block-
ing them and retarding in such a way propane activation.
At high temperatures, when propane can be selectively ac-
tivated on silver, the level of NOx conversion is enhanced
due to suppression of a deep oxidation route.
When interaction between silver and oxidic copper clus-
ters is pronounced, which is achieved by using a special
preparation procedure, it helpsto enhance performance, es-
pecially in the low- and middle-temperature ranges, mainly
due to the following reasons:
Analysis of the data obtained here allows preliminary
conclusions concerning the role of oxidic (copper) and
metallic (silver) components in the reactions of NOx re-
duction by hydrocarbons to be drawn.
When the interaction between silver and copper oxidic
clusters is negligible, silver addition does not effect the cat-
alyst performance in the NOx reduction. This is explained
as follows:
(i) Due to decrease of the bonding strength of oxygen
located on mixed oxidic clusters, the rate of hydrocarbon
activation is increased.
(ii) In mixed clusters, the bonding energy of nitrite–
nitrate species is changed in a nonadditive manner, so that
population of forms with the intermediate bonding strength
is usually increased, while that of a high bonding strength-
decreased.
(i) For catalysts containing copper cations, the rate of
NO oxidation into NO2 then converted to reactive nitrite–
nitrate species is high (vide supra), so silver addition does
not affect it. As a result, contrary to the case of sil-
ver/alumina system (10), the surface coverage by reactive
intermediates including nitrite–nitrate species and organo-
NOx compounds is expected not to change noticeably.
(ii) The rate of hydrocarbon activation/oxidation is also
not expected to be increased in the case of a weak (if any)
interaction between silver and copper oxidic species. In-
deed, the highest rates of hydrocarbon oxidation are typ-
ical for three-dimensional copper oxidic clusters fixed on
support due to decreased bonding strength of oxygen (18).
As follows from our data for zirconia-supported copper ox-
3. The Role of the Type of Reducing Agent
For the systems considered here, in the first approxima-
tion, for different reducing agents, the orders of activity
were rather similar. The most important feature of samples

138
SADYKOV ET. AL.
studied here is that they ensure a high (up to 70% ) level of mally stable bridging nitrates can be bound simultaneously
NO conversion into N2 at rather low (350–370 C) temper- with copper and zirconium cations as well as with the guest
atures in the excess of oxygen using propane as reductant. cations such as Ba located at the surface (7). Certainly, the
Thislow-temperature level of performance approachesthat decrease of the copper–oxygen bonding strength due to sil-
for the overexchanged sample of Cu-ZSM-5 ( 2 wt% of ver incorporation into copper oxidic clusters also helps to
Cu, Si/Al2 50) (20) tested earlier with the same reductant decrease the thermal stability of nitrates (vide supra), and,
and the same feed composition ( 100% NO conversion hence, their steady-state coverage at a given temperature,
into N2 at 300–330 C). Though the highest level of activ- thus enhancing the rate of hydrocarbons activation.
ity was obtained here for silver-promoted samples, a good
In the case of more reactive propylene as reductant, its
NOx conversion (up to 50% ) was also observed for sam- activation is known to be less demanding; hence, copper
ples without silver. Earlier (1–3, 6), mixed copper–zirconia cation clustering degree or the surface site blocking by
oxide samples were demonstrated to be rather active in strongly bound nitrates appear to be less important for the
the reaction of NO-selective reduction by propylene (NO low-temperature performance. Asthe result, sampleswith a
conversion up to 70% at GHSV 10,000/h), while being lowcopper loadingprepared via cation exchange have iden-
only moderately active with propane as reductant (conver- tical maximum values of NOx conversion, which are even
sion not exceeding 10% at 450 C). This difference clearly somewhat higher than those for some silver-promoted sam-
implies that any comparison between the performance of ples with a higher copper loading. In this respect, our results
catalysts in NOx-selective reduction by different hydrocar- agree with conclusions of Pietrogiacomi et al. (7) about the
bon should take into account not only their chemical com- highest turnover rates in the NOx selective reduction by
position but the microstructure as well. In our case, partially propylene for isolated copper cations dispersed on mon-
stabilized mesoporous cubic zirconia samples are used as a oclinic zirconia support. However, we must also keep in
support, and distribution of copper cations between species mind that for the route of NOx reduction by propylene pro-
with a different copper coordination and clustering degree ceeding via No interaction with the coke deposited on sup-
certainly differs with that for previously studied coprecipi- port (11, 22, 23), any relation between the catalyst perfor-
tated copper–zirconia samples (1–3). Besides, it is affected mance and characteristics of a “clean” surface hardly could
by the nature of stabilizing alkaline-earth cations as well be straightforward. At low temperatures, coke can block
(7). Similar effects appear to take place in the case of cop- the surface, thus decreasing performance of catalysts with
per supported on the sulfated zirconia surface. In this case, a low oxidizing ability. Indeed, silver addition to CuO/BaO–
the maximum level of NOx-selective reduction by propane ZrO2 system was found to make it the most active catalyst in
was increased up to 40% , though being shifted to higher the reaction of NOx reduction by propylene in a very broad
( 500–550 C) temperatures (21).
(150–500 C) temperature range. It implies that decreasing
In the case of NO-selective reduction by propane on the oxygen bonding strength with copper–silver oxidic clus-
copper-containing catalysts, breaking of the C–H bond is ters is favorable for a good catalysts performance in this
considered to be the rate-determining stage followed by reaction as well.
interaction of the activated fragment with nitrite–nitrate
For the reaction of NOx reduction by decane, the
species yielding an organic nitrocopound (13, 16, 17). This low-temperature performance of copper/zirconia samples
stage can be facilitated either by increasing the coordina- not promoted by silver is as good as that seen with
tion unsaturation of the copper cations or by increasing Cu/mordenite (24). The absence of activity for some
the surface oxygen reactivity via decreasing its bonding samples reasonably active with propane and propylene as
strength. In addition, the bonding strength of reactive reductants suggests the surface blocking by coke and/or car-
intermediates—nitrate–nitrate species—is known to de- bonates. Indeed, strong deactivation of BEA zeolitic cata-
crease with decreasing surface oxygen bonding strength, lysts in the decane NOx SCR due to coking was earlier
while corresponding reactivity of these species increases observed by Delahay et al. (25). Among inactive samples,
(9). Another important factor is that coordinatively unsat- those with a low density of coordinatively unsaturated Cu+
urated sitescan be blocked in the reaction media bystrongly centers are present, thus suggesting their importance for
bound nitrate species, thus hindering hydrocarbon activa- the decane activation as well. However, performance is
tion. The last factor appears to be decisive for the low- greatly improved by the silver addition, which helps thus
temperature performance of samples studied here in the to approach the maximum level of NOx reduction by de-
reaction of NOx-selective reduction by propane: the high- cane at moderate (300–350 C) temperatures similar to that
est level of NOx conversion was obtained for 0.2% CuO / for copper/sulfated zirconia (4, 5), Cu/mordenite (24), or
SrO–ZrO2 and 2% Ag + 5% CuO /BaO–ZrO2 sample with copper-exchanged beta zeolite (25). In the case of de-
the lowest surface coverage by nitrate species (7). For cata- cane as reducing agent, sample modification by silver via
lysts with a low copper content prepared via the cation ex- a special impregnation procedure helps to increase the sur-
change, for which isolated copper cations dominate, ther- face density of Cu+ centers due to bulky oxidic cluster

HC-SCR OF NOx ON CuO (+Ag)/MeO–ZrO2
139
redispersion and/or formation of mixed copper–silver
oxidic microphases. Silver addition appears also to be im-
portant for decreasing the surface blocking by coke via fa-
cilitation of its oxidation by weakly bound oxygen (25).
Certainly, further studies are required to prove such a sug-
gestion.
Since the highest level of low-temperature activity in all
three reactions was achieved by the same Ag + CuO/BaO–
ZrO2 sample with the highest degree of interaction between
copper and silver oxidic species (7), such an interaction
appears to be important for ensuring a good performance
in NOx selective reduction by different hydrocarbons.
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CONCLUSIONS
In the reaction of NOx selective reduction by hydrocar-
bons (propane, propylene, and decane), catalytic perfor-
mance of zirconia-suported copper oxidic species including
those promoted by silver appears to be mainly determined
by their local structure and composition affecting the bond-
ing strength of oxygen and strongly bound nitrate species.
In general, for the same system, the maximum level of NOx
conversion and temperature of maximum NOx conversion
is strongly dependent upon the type of reducing agent. No
universal relation was found between the surface density
of coordinatively unsaturated cations able to activate hy-
drocarbons and the activity of catalysts for the HC-SCR. It
can be explained by blocking the surface sites in reaction
media either by strongly bound nitrate species or by coke.
Addition of silver was found to be of significance only in
the case of strong interaction between the metallic and ox-
idic components. Such an interaction helps to decrease the
bonding strength of oxygen with mixed copper–silver oxidic
species, thus decreasing the bonding strength of adsorbed
nitrate species. When it takes place, in all three reactions
studied here, the level of activity at moderate (250–350 C)
temperatures approaches that for the most efficient copper-
containing systems tested up to date such as Cu-ZSM-5,
Cu/sulfated zirconia Cu/mordenite, or Cu-beta zeolite.
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V. V., Matyshak, V. A., and Rozovskii, A. Ya., React. Kinet. Catal. Lett.
65, 113 (1998).
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ACKNOWLEDGMENTS
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92, 5129 (1996).
This research was supported by the INTAS Project 97-11720.
23. Vergne, S., Berreghis, A., Tantet, J., Canaff, C., Magnoux, P., Guisner,
M., Davias, N., and Noirot, R., Appl. Catal. B 18, 37 (1998).
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